Polycrystalline diamond cutters, also known as Polycrystalline Diamond Compacts (PDCs), are made from synthetic diamond or natural diamond crystals mounted on a substrate made of tungsten carbide. The sintering process used to manufacture these devices typically begins with premium saw-grade diamond crystals. The diamond crystals are sintered together at temperatures of approximately 1400° C. and pressures of around 61 kbar in the presence of a liquid metal synthesizing catalyst, most commonly cobalt, functioning as a binder. Other catalysts can be used including elements from the Group VIII metals (as well as alloys of Group VIII metals), silicon, and other alloys such as magnesium carbonate. The temperature of 1400° C. is typically maintained for approximately 5 to 10 minutes. The system is then cooled and finally depressurized. The pressure rate, the heating rate and the cooling rate depend on the type of equipment (belt or cubic press) used, the particular catalyst used and the raw-grade diamond crystals used. Typically, the diamond is bonded to the tungsten carbide substrate during the same high-temperature, high-pressure process.
It is commonly recognized that PDC cutters wear according to three different modes characterized by the temperature at the cutter tip (see, Ortega and Glowka, “Studies of the Frictional Heating of Polycrystalline Diamond Compact Drag Tools During Rock Cutting,” June 1982; and Ortega and Glowka, “Frictional Heating and Convective Cooling of Polycrystalline Diamond Drag Tools During Rock Cutting,” Soc. of Petr. Eng. Journal, April 1984; the disclosures of which are hereby incorporated by reference). Below 750° C., the primary mode of wear is micro-chipping of the sintered diamond. Above 750° C., the wear mode changes from micro-chipping of individual diamond grains to a more severe form of wear. This more severe form of wear is caused by 1) stresses resulting from differential thermal expansion between the diamond and the residual metal inclusions along the diamond grain boundaries, and 2) a chemical reaction of the diamond to the cobalt turning the diamond back to graphite as it approaches 800° C.
The prior art teaches a way to extend cutter life by removing the cobalt catalyst from the PDC diamond table to a depth of less than 100 μm, or perhaps between 100 to 200 μm or more, using an acid attack. The acid leaches out substantially all of the interstitial cobalt from the face of the diamond layer to the desired depth leaving interstitial openings. This treatment suppresses the potential for differential thermal expansion between the diamond and the catalyst metal and increases the thermal diffusivity at least in the area of the leached depth from a front face of the diamond table. These products are known to those skilled in the art as leached PDCs and they have an industry recognized performance improvement over non-leached PDCs. The acids required by the leaching process can be harsh and difficult to handle safely.
Leached PDC cutters have been considered to have improved performance over non-leached cutters because of several reasons:
First: The absence of interstitial cobalt in a thermal channel situated along the front face of the diamond table improves heat transfer to drilling fluid, across the diamond table face and to the interior of the cutter through presence of diamond to diamond bonding. Heat transfer along the thermal channel helps to keep the temperature at the cutter tip below a critical temperature past which failure due to diamond chipping occurs. This is due at least in part to the absence of a substantial differential thermal conductivity characteristic (note: a 2000 Wm−1K−1 thermal conductivity for the diamond in comparison to a 60 Wm−1K−1 thermal conductivity for cobalt). Additionally, while the cobalt has been removed and replaced by a void in the interstices of the leached cutter, the void (which also has poor heat dissipation characteristics) nonetheless appears to create less interference with respect to dissipation of heat across the diamond to diamond bonds than is experienced when interstitial cobalt is present. This explains to some degree why leached cutters perform better than non-leached cutters.
Second: The region where the cobalt has been removed does not appear to suffer bond breakage due to cobalt thermal expansion. This is due at least in part to the absence of a substantial differential thermal expansion characteristic (note: a 13 μmm−1K−1 thermal expansion coefficient for cobalt in comparison to a 1 μmm−1K−1 thermal expansion coefficient for diamond). This second point has, according to conventional wisdom, been the key reason for the success of leached PDC cutters.
Third, the heat capacity of the thermal channel situated along the front face of the diamond table decreases which results in a substantial improvement in thermal diffusivity.
Notwithstanding the foregoing and the advantage of leached cutter technology, there exists a need in the art for improvement by providing a leached PDC cutter possessing better thermal properties.
Reference is made to the following prior art documents: U.S. Pat. Nos. 4,016,736; 4,124,401; 4,184,079; 4,605,343; 4,940,180; 5,078,551; 5,609,926; 5,769,986; 5,857,889; 6,779,951; 6,887,144 and 7,635,035; Published PCT Application WO 01/79583; Wang, “A Study on the Oxidation Resistance of Sintered Polycrystalline Diamond with Dopants,” Science and Technology of New Diamond, pp 437-439, 1990; Salvadori, “Metal Ion Mixing in Diamond,” Surface and Coatings Technology, June 2000, p. 375; Pu, “The Application of Ion Beam Implantation for Synthetic Diamond Surface Modification,” IEEE Int. Conf. on Plasma Science, 1197; Weishart, “N-type Conductivity in High-fluence Si-implanted Diamond,” Journal of Applied Physics, vol. 97, issue 10, 2005; Vankar, “Ion Irradiation Effects in Diamond and Diamond Like Carbon Thin Films,” 1995; Dearnaley, “The Modification of Material by Ion Implantation,” Physics in Technology 14, 1983; Stock, “Characterization and Mechanical Properties of Ion-implanted Diamond Surfaces,” Surface and Coatings Technology, vols. 146-147, 2001; “Modification of Diamond Single Crystals by Chromium Ion Implantation with Sacrificial Layers,” Analytical and Bioanalytical Chemistry, vol. 374, nos. 7-8, 2002; the disclosures of which are hereby incorporated by reference.